Brain development hypothesis challenged

A world-first study into mammalian brain development, led by researchers from the University of Queensland, has shown that the controversial “late equals large” hypothesis simply does not add up.

Taking advantage of the “huge diversity of animals that Australia has to offer”, Dr Vera Weisbecker of the University of Queensland’s School of Biological Sciences has shown that mammalian brain development does not fit the universal model proposed in the 1990s, with significant variance seen across different species.

Dr Weisbecker’s research was designed to tell the evolutionary story of the enormous variety of land vertebrates known today, through developmental biology: “Our results using information from brain growth in marsupial mammals also provide a new test of the long-debated ‘late equals large’ hypothesis first published in 1995, to explain the way brains evolve across species.”

This is the first study to compare brain development in different mammalian species, using DiceCT — a novel method of non-invasive micro-computed tomography scanning. This method allows for fast data acquisition of soft tissue growth in tiny mammals, which can be widely applied in areas as diverse as evolutionary biology, congenital defects and biomedical investigation into brain development.

“Instead of one neurogenesis-based rule, we suspect that the evolution of brain parts, including the huge human cerebral hemispheres, results from a complex combination of factors including the early molecular processes which divide the brain long before it starts growing.”

Previously it was presumed that the development of the cerebral hemispheres seen in humans was a side effect caused by the larger size of our brains forcing different portions to specialise. This theory was a result of the idea that larger brain parts develop later and longer from the neural stem cells.

Dr Weisbecker said: “For these cell-level ‘rules’ to be translated into specific brain proportions, we would also expect to see these rules reflected in predictable growth patterns of the mammalian brain, particularly in species from the same group of mammals. Our research looked for such common patterns in brain development by providing the first data on brain growth for three species of marsupial mammals and the results show that this hypothesis does not work.”

This research marks the first time that mammalian brain development has been quantified across several species.

“In addition, when we compared adult brain proportions, we saw that the relationship between brain proportions and size depends on what group of mammals we look at, which is also incompatible with a universal rule for brain proportions.”

Published in Scientific Reports, this research used the resources of the CSIRO Health and Biosecurity Flagship, the University of Melbourne and the University of Queensland’s Centre for Advanced Imaging.

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